Young people under the age of 25 are at great risk for sexually transmitted diseases (STDs) including the human immunodeficiency virus (HIV) (CDC, 2004). Further, the highest rates of many common STDs occur in young people between the ages of 15 and 24 (CDC, 2000). Adolescents aged 13-19 are the only age group for whom the number of newly diagnosed HIV cases in women (64%) exceed the number of newly diagnosed men (36%). Adolescents involved with the criminal justice system exhibit high levels of risky sexual behavior, resulting in high rates of unintended pregnancy and STDs among criminally-involved adolescents. Marijuana use is particularly common among criminally-involved youth, and has been characterized in the RFA to which this application responds as an "emerging drug of abuse in relation to HIV risk for highly vulnerable youth" (DA-04-012). Recent research supports this claim, finding stronger associations between marijuana use and risky sexual behavior than between alcohol use and risk sex among high risk adolescents (Kingree et al., 2000; Kingree & Betz, 2003). The goal of the proposed research is to utilize a two-year longitudinal design to increase our understanding of the relationship of marijuana use to risky sexual behavior among highly vulnerable youth aged 14-17 years, who are using or are at high-risk for using marijuana, and who are vulnerable to sexually transmitted diseases including HIV. In addition, the proposed research will begin to fill a crucial gap in research on adolescent HIV risk behavior - the role of gender in sexual risk behaviors and the potential moderating role of gender in the marijuana/risky sex connection. The proposed research is expected to have significant implications for the development and testing of theory-based and empirically targeted gender-specific HIV prevention research will lay the foundation for future work that seeks to develop an intervention that specifically targets the differential social and psychological barriers to condom use experienced by adolescent males and females, and includes targeted content focused on the relationship of high levels of marijuana use to risky sex in this population.